Cardiac arrhythmias, e.g., fibrillation, are irregularities in the normal beating pattern of the heart and can manifest themselves in either the atria or the ventricles of the heart. For example, atrial fibrillation is a form of arrhythmia characterized by rapid randomized contractions of atrial myocardium, causing an irregular, often rapid ventricular rate. The regular pumping function of the atria is replaced by a disorganized, ineffective quivering as a result of chaotic conduction of electrical signals through the upper chambers of the heart. Atrial fibrillation is often associated with other forms of cardiovascular disease, including congestive heart failure, rheumatic heart disease, coronary artery disease, left ventricular hypertrophy, cardiomyopathy, or hypertension.
Effective catheter treatment of atrial fibrillation was made possible by a breakthrough discovery in the late 1990's by investigators in Bordeaux, France. They found that recurrent atrial fibrillation (paroxysmal and persistent) is triggered by rapidly firing tissue, (called “ectopic foci”), that are located in one or more of the four pulmonary veins, which attach to the rear of the left atrium. Their research, since confirmed by others, suggested that 85% or more of the ectopic foci that initiate atrial fibrillation are located in or at the ostium, (mouth), of the pulmonary veins. They demonstrated that atrial fibrillation could be cured by electrically isolating the pulmonary veins from the rest of the atrium.
Various techniques have been proposed for pulmonary vein isolation. Although these procedures were originally performed with a scalpel, various other techniques have also been developed to form lesions. Collectively, these treatments are referred to as “ablation.” In non-surgical ablations, the tissue is treated, generally with heat or cold, to cause coagulation and/or tissue necrosis (i.e., cell destruction). In each of these techniques, cardiac muscle cells are replaced with scar tissue which cannot conduct normal electrical activity within the heart.
In one known approach, circumferential ablation of tissue within the pulmonary veins or at the ostia of such veins has been practiced to treat atrial fibrillation. By ablating the heart tissue at selected locations, electrical conductivity from one segment to another can be blocked and the resulting segments become too small to sustain the fibrillatory process on their own.
Several types of ablation devices have recently been proposed for creating lesions to treat cardiac arrhythmias, including devices which employ electrical current (e.g., radio-frequency (“RF”)) heating or cryogenic cooling. Such ablation devices have been proposed to create elongated lesions that extend through a sufficient thickness of the myocardium to block electrical conduction. Many of the recently proposed ablation instruments are percutaneous devices that are designed to create such lesions from within the heart. Such devices are positioned in the heart by catheterization of the patient, e.g., by passing the ablation instrument into the heart via a blood vessel, such as the femoral vein.
Devices that rely upon resistive or conductive heat transfer can be prone to serious post-operative complications. In order to quickly perform an ablation with such “contact” devices, a significant amount of energy must be applied directly to the target tissue site. In order to achieve transmural penetration, the surface that is contacted will experience a greater degree of heating (or freezing). For example, in RF heating of the heart wall, a transmural lesion requires that the tissue temperature be raised to about 50° C. throughout the thickness of the wall. To achieve this, the temperature at the contact surface will typically be raised to greater than 100° C. In this temperature regime, there is a substantial risk of tissue destruction (e.g., due to water vaporization micro-explosions or due to carbonization). Charring of the surface of the heart tissue, in particular, can lead to the creation of blood clots on the surface and post-operative complications, including stroke. Even if structural damage is avoided, the extent of the lesion (i.e., the width of the ablated zone) on the surface that has been contacted will typically be greater than necessary.
Cardiac ablation instruments also suffer from a variety of design limitations. For example, the shape of the heart muscle adds to the difficulty in accessing cardiac structures, such as the pulmonary veins on the anterior surface of the heart. Typically, percutaneous devices are positioned with the assistance of a guide wire, which is first advanced into heart. In one common approach, described, for example, in U.S. Pat. No. 6,012,457 issued to Lesh on Jan. 11, 2000 and in International Application Pub. No. WO 00/67656 assigned to Atrionix, Inc., a guide wire or similar guide device is advanced through the left atrium of the heart and into a pulmonary vein. A catheter instrument with an expandable element is then advanced over the guide wire and into the pulmonary vein where the expandable element (e.g., a balloon) is inflated. The balloon includes a circumferential ablation element, e.g., an RF electrode, carried on the outer surface of the balloon, which performs the ablation procedure. The balloon must be large enough and sufficiently rigid to hold the electrode in contact with the inner surface of the pulmonary vein for the length of the procedure. Moreover, because the lesion is formed by an ablation element carried on the surface of the balloon element, the balloon shape inherently limits the locations where a lesion can be formed, i.e., the lesion must be formed at least partially within the pulmonary vein.
In another approach described in U.S. Pat. No. 6,235,025 issued to Swartz et al. on May 22, 2001, a guide wire is again used to percutaneously access a pulmonary vein and a catheter is again slid over the guide wire to a position within the pulmonary vein. The catheter device includes two spaced-apart balloons, which are inflated in the vein (or in the vein and at its mouth). The space between the two balloons can then be filled with a conductive fluid to delivery RF energy (or, alternatively, ultrasound) to the vein and thereby induce a conduction block in the blood vessel by tissue ablation. With the Swartz et al. device, like the Lesh device, the region where tissue ablation can occur is limited by the design. Because two balloons must seal a space that is then filled with an ablative fluid, the lesion is necessarily formed within the pulmonary vein.
Ablation within the pulmonary vein can result in complications. Overtreatment deep within a vein can result in stenosis (closure of the vein itself), necrosis or other structural damage, any of which can necessitate immediate open chest surgery.
A limitation of these commonly utilized instruments is the lack of site selectability. Practically speaking, each such percutaneous instrument is inherently limited by its design to forming an ablative lesion at one and only one, location. For example, when an expandable balloon carrying an RF heating device on its surface is deployed at the mouth of a vein, the lesion can only be formed at a location defined by the geometry of the device. It is not possible to form the lesion at another location because the heating element must contact the target tissue. Similarly, the above-described tandem balloon device can only form a lesion at a location defined by the space between the balloons that is filled with the ablative fluid.
Another limitation of such known instruments and methods is their inability to accommodate the varied geometry of the heart. For example, the inner surface of the atrium is not regular. In particular, the mouths of the pulmonary veins do not exhibit regularity; they often bear little resemblance to conical or funnel-shaped openings. Thus, when the above-described expandable, contact heating devices encounter an irregularly-shaped ostia, the result can be an incompletely formed (non-circumferential) lesion.
Moreover, the size or shape of the pulmonary vein ostia that are encountered may be too big or too small for the selected balloon catheter and it may be necessary to remove the first balloon catheter from the patient and replace it with another instrument having a balloon element of a different size. Replacement of the catheter with another before a procedure can begin (or in the middle of a multiple vein ablation protocol) can substantially increase the overall duration of the procedure and/or increase the chance of trauma.
Accordingly, there exists a need for better cardiac ablation instruments that can quickly and effectively create pulmonary vein encircling lesions even in the face of irregularly shaped or variable sized target tissue regions.